adopt `@generated` marker for generated `*.Rd` files

[maxheld83]

instead of:

% Generated by roxygen2: do not edit by hand

it should read:

% @generated by roxygen2: do not edit by hand

This is the https://generated.at standard.

It's meant for specifically the roxygen2 situation: Generated files which (regrettably) get committed to git (~~.gitignore~~).

Potential consumers of this information might be linters, editors and other dev tooling.

Happy to add a PR to this effect, since that should be quite trivial.

Only issue: should this ever be adopted, a lot of people would have one-off spurious diffs in their man/, which would be a nuisance.